What it means
だけでなく means “not only ... but also” or “as well as.” It adds a second item, quality, or action to the first one and shows that both are true. The added part commonly uses も, although も can be omitted when the context is clear. This pattern can connect nouns, adjectives, and clauses, and it is suitable for both everyday speech and writing.
Usage note
With nouns and na-adjectives, use だけでなく and なだけでなく respectively. With verbs, the clause before だけでなく is normally in the plain form, even when the main sentence is polite. Do not confuse だけでなく with だけでなくて, which has a different connective use.
How to use it
この店は野菜だけでなく、果物も売っています。
This shop sells not only vegetables but also fruit.
この図書館は静かなだけでなく、席も多いです。
This library is not only quiet but also has many seats.
この道は長いだけでなく、危険です。
This road is not only long but also dangerous.
兄は毎朝走るだけでなく、夜も泳ぎます。
My older brother not only runs every morning but also swims at night.
この店は野菜だけでなく、果物も売っている。
This shop sells not only vegetables but also fruit.
この図書館は静かなだけでなく、席も多い。
This library is not only quiet but also has many seats.
この道は長いだけでなく、危険だ。
This road is not only long but also dangerous.
兄は毎朝走るだけでなく、夜も泳ぐ。
My older brother not only runs every morning but also swims at night.